OPTIMIZATION OF A DISTRICT ENERGY SYSTEM IN ZARAGOZA
OPTIMIZATION OF A DISTRICT ENERGY SYSTEM IN ZARAGOZA
OPTIMIZATION OF A DISTRICT ENERGY SYSTEM IN ZARAGOZA (SPAIN) Natalia Moreno Bruned June 2009 Master’s Thesis in Energy Systems Supervisor: Alemayehu Gebremedhin Examiner: Alemayehu Gebremedhin DEPARTMENT OF TECHNOLOGY AND BUILT ENVIRONMENT
Preface This project has been carried out as the final thesis of the Master’s Programme in Energy Systems at the University of Gävle (Sweden). Firstly, I would like to give my most sincere thanks to my lecturer Miguel Ángel Lozano from the University of Zaragoza (Spain) for the excellent guidance given to me throughout the development of the thesis.
This project has been possible thanks to his involvement in the research work. His support and advice was most helpful. I would also like to thank my supervisor at the University of Gävle, Alemayehu Gebremedhin, whose attention, comments and advice have been of great help. Furthermore, I would like to thank all the other people involved in this project for their support and cooperation who have not been mentioned. Finally, I would like to give special thanks to my family, my friends and my boyfriend Ángel; whose moral support and understanding during my study years has been a great help. They encouraged me when I decided to study abroad for one year because they knew that it would be an incredible experience for my personal, professional and human enrichment.
Thank you, Natalia Moreno Bruned
Abstract The main objective of the present thesis is to study the design and optimization of a district energy system. The system is designed to meet the energy demand of a housing development located in the city of Zaragoza (Spain). The trigeneration system supplies domestic hot water, heating, cooling and electricity to the community. The area is supposed to be composed of a number of residential buildings, a hospital, a hotel and a school. The energy needs are provided by a CHP plant combined with several cooling machines.
The selected equipment as well as the way of operation has been decided upon by using an optimization program called MODEST (Model for Optimisation of Dynamic Energy Systems with Time dependent components and boundary conditions). MODEST is a model based on linear programming which is used for optimizing energy systems. The program optimizes the energy system in order to meet the three types of demands within the area: heating, cooling and electricity. The result of the optimization gives the most cost-effective combination of equipment and fuels throughout the year to meet the energy demand.
Furthermore, district energy systems provide a good solution to take advantage of the installed power in the CHP plant. Cogeneration systems combined with absorption machines enable the excess heat from electricity production, which is often a problem during the warm season, to be used. This waste heat fuels absorption chillers to generate cooling. However, the cooling capacity needed in some periods is higher than the power required for the production of heat. Thus, it is more suitable to combine absorption and compression cooling possibilities. The problem has been solved by comparing the optimisation results of two different scenarios: first, considering the option of installing a gas turbine CHP combined with a small gas engine CHP and second the assumption of only gas engine CHP technology.
Both systems have been compared from energy and economic perspectives. The final conclusion is that the scenario composed only of gas engine CHP is much more efficient as well as being a bit cheaper annually. Moreover, environmental effects are also taken into account when the decision is made.
Broadly speaking, the introduction of a district energy system in Spain offers a great opportunity to promote the use of efficient technology in the residential sector by installing CHP systems combined with absorption possibilities. These systems provide a big potential to decrease greenhouse gases emissions; contributing to the fight against climate change. However, an important effort by the authorities will be necessary to support district systems and to change the current attitudes of society. Citizens must be made aware of environmental problems and real needs of using renewable and efficient energy.
Abbreviations AM Absorption Cooling Machine BOE Boletín Official del Estado (Official State Bulletin) CHP Combined Heat and Power CHCP Combined heating, cooling and power generation CM Compression Cooling Machine CNE Comisión Nacional de la Energía (Nacional Energy Comisión) COP Coefficient of Performance CTE Código Técnico de la Edificación DC District Cooling DH District Heating BB Biomass boiler GB Gas Boiler GT Gas Turbine GE Gas Engine IDEA Instituto para la Diversificación y Ahorro de la Energía MODEST Model for Optimization of Dynamic Energy Systems with Time dependent components and boundary conditions MPa Megapascal MW Megawatt MWh Megawatt Hour O&M Operating and Maintenance OMEL Operador del Mercado de Energía RD Royal Decree REE Equivalent Electric Efficiency TOE Ton Oil Equivalent
Table of contents 1. Introduction . 1 1.1. Background . 1 1.2. Objective . 1 1.3. Limitations . 1 2. Literature study . 2 2.1. District heating and district cooling . 2 2.1.1. District heating . 2 2.1.2. District cooling . 4 2.1.3. Environmental effects . 4 2.1.4. Situation of district energy system in Spain . 5 2.2. Cogeneration and trigeneration . 7 2.2.1. Cogeneration . 7 2.2.2. Trigeneration . 10 2.2.3. Efficiency parameters of cogeneration Systems . 11 2.2.4. Environmental and economic advantages . 12 2.2.5. Development of cogeneration in Spain . 12 2.3. Combined Heat and Power technology .
18 2.4. Cooling techniques: Compression and absorption systems . 20 2.4.1. Compression cooling . 20 2.4.2. Absorption cooling . 21 2.5. Biomass in Spain . 24 2.6. Current heating and cooling systems in Spain . 26 2.7. Electricity market in Spain: Special Regime . 29 3. Method . 31 3.1. Gathering of facts . 31 3.2. Gathering of data . 31 3.3. MODEST software . 32 3.4. Scenario modelled . 33 3.5. Sensitivity analysis . 33 3.6. Limitations . 34 4. System description . 35 4.1. Location of the urbanization . 35 4.2. Description of the urbanization . 36 4.3. System model . 37 4.4. Input data . 38 4.4.1.
Demand characteristics . 38 4.4.2. Equipment prices and characteristics . 42 4.4.3. Fuel prices . 42 5. Results of the optimization . 44 5.1. Scenario modelled . 44 5.1.1. Components of the system . 44 5.1.2. Energy consumption . 45 5.1.3. Production of heating, cooling and electricity . 46 5.1.4. Duration graphs . 49 5.1.5. CO2 emissions . 50
5.1.6. System cost . 50 5.1.7. Alternative operation . 50 5.1.8. Calculation of the REE . 51 5.1.9. Energy analysis . 51 5.1.10. Economic analysis . 52 5.1.11. Sensibility analysis . 53 5.2. Alternative scenario . 54 5.2.1. Components of the system . 55 5.2.2. Energy consumption . 56 5.2.3. Production of heating, cooling and electricity . 58 5.2.4. Duration graphs . 60 5.2.5. CO2 emissions . 61 5.2.6. System cost . 61 5.2.7. Alternative operation . 62 5.2.8. Calculation of the REE . 62 5.2.9. Energy analysis . 62 5.2.10. Economic analysis . 63 6. Discussion and conclusion . 65 References .
68 Appendixes . 72 Appendix A . 72 Appendix B . 75 Appendix C . 76
1. Introduction 1. Introduction This chapter gives an introduction to the issue of the thesis. The background describes why the issue is interesting to investigate; followed by the main purpose of the analysis. Further on in this section a brief explanation of the main limitations found during the work is given. 1.1. Background In Spain, the current situation is that heating and cooling demand is met by individual systems. In the great majority of the cases, boilers and small air conditioning equipment are installed in each dwelling or building. This results in high fuel and electricity consumption.
The idea of the thesis is to study the possibility of installing district energy systems that supply energy to a residential area. This type of system has been widely used around the world but its introduction has been slow in Spain, where only a few of these systems have been installed. The intention of district energy systems is that the energy is produced centrally and then distributed to all the users in the area through a shared infrastructure. In this way, it is possible to use efficient technology such as cogeneration systems, thereby decreasing fuel consumption and environmental effects.
On the other hand, the climatic change due to the increase of greenhouse gases in the atmosphere is an important preoccupation for Spanish authorities. They support renewable energy and efficient technology concepts by means of economic incentives. 1.2. Objective The main objective of the thesis is to promote the introduction of district energy systems in Spain as a way of replacing current individual systems. The present work analyses the possibility of installing a central system that produces heating, cooling and electricity to meet the energy demand of a residential area composed of diverse types of building with different final functions.
The thesis is based on the design and optimization of a district energy system located in Zaragoza (Spain). The energy system is optimised by using an optimization program called MODEST that helps to decide the type of components and fuels used as well as the most profitable method of operating. Furthermore, it is desirable that the system is efficient and environmental friendly.
1.3. Limitations The first limitation of the work was to find reliable input data about the several types of energy demand of the considered buildings. It was a complicated task involving a great deal of time because it is not usual to register the hourly energy consumption of a place. The types of buildings included in the urbanization have been defined taking into consideration the available input data. Furthermore, it proved extremely difficult to collect all the input data about the technology and fuels used in these types of systems as well as to adapt them for use in MODEST software.
Finally, it was necessary to build a simulation model, to overcome various problems, which were ultimately resolved by this means. More limitations and assumptions made during modelling are described in detail in the section 3.6. Limitations.
2. Literature study 2 2. Literature study The study of available literature is an important aspect in order to acquire a deep knowledge about the topic to be developed during this thesis. A detailed study must be done in order to be able to analyse the situation and to make conclusions. This section presents the literature that has been studied to make the evaluation. First, a description of DH and DC systems is given followed by an explanation about cogeneration and trigeneration. Next, the CHP technologies and the different cooling techniques are analysed.
2.1. District heating and district cooling District heating and cooling systems are thought to be one of the most outstanding ways to maximise the efficiency of processes that are used to produce heating and cooling.
They allow an electricity generation process to be optimised by using CHP plants that save energy. Furthermore, different types of fuels can be used to produce it, increasing fuel flexibility and providing opportunities for introduction of renewable energy sources as well as industrial waste heat.
2.1.1. District heating District heating represents a type of energy which is thought to be cleaner. It is produced in an efficient and cost-effective way. Heat generation is centralized and DH is delivered to residential homes and commercial buildings in a certain area. DH systems can provide both space heating and hot water. [IEA DHC, 2009] District heating uses water that is centrally heated and distributed through a pipe-system to individual users in areas of high concentration of activities and housing. In large cities several thousands of users can be connected to the system. In DH systems, heat is supplied to all customers in the city through a supply pipe in the ground and distributed to all substations of the houses which are connected in parallel between the supply pipe and the return pipe.
DH delivers heat in services pipes to the house heating system and to the tap hot-water system by means of district-heating substations. The water then returns to the plant to be heated again. Any district heating network is composed of three main sectors: production, distribution and market (customers). [IEA DHC, 2009] District heating presents a number of advantages which have contributed to its quick development all over the world. It offers flexibility for heat production and enables combined heat and power production. Technically, larger system results in better efficiency and lower specific costs.
What is more, the plant can use many types of fuels; it is possible to use local energy sources, such as biogas or wood fuel. It can also absorb low grade heat sources such as garbage burning or industrial waste heat, which are released to the environment in countries that do not use DH system. Thus, it is an environmentally friendly way to heat houses, buildings and schools. [Danfoss, 2009] DH network is made up of with steel pipes with a compound integrated plastic jacket (jacket pipes). Pipes have to be installed in order not to move too much in the ground when they are heated up. In smaller systems, pipes make of flexible copper or plastic can be used, which are more simple and cheaper to install.
However, plastic pipes have a limited pressure of 6 Bar and temperature of 90°C.
2. Literature study 3 The temperature of supply of DH depends on the expected load, which is a function of the outdoor temperature. Figure 1 shows typical DH-temperatures in supply and return pipes. Figure 1: District heating temperatures. Source: Zinko, H., 2008 Some typical design values in northern countries can be pressure of 1,6 MPa (16 Bar), temperature of approximately 120 ºC (130 – 140 ºC) and flow velocity from 1 m/s in smaller pipes to 2 m/s in larger pipes. The pressure of the water in the pipes drops due to the friction in the pipes. Therefore, the pressure difference is measured at a distant point away from the plant.
If the pressure difference is too small, the pump is ordered to deliver a higher pressure. Control valves in the substations control the flow to each load and temperature sensors at the customer demand side control the amount of flow which is necessary in order to heat the water to the desired temperature. The temperature of radiators which is the temperature of the DH supply pipe is controlled according to the outdoor temperature. At the heating plant, there is only one pump which pumps all the water needed by the whole city. [Zinko, H., 2008] Heat losses of a district heating system are presented in Figure 2.
The most significant losses are due to distribution of DH.
• Production losses 8 - 12 % • Distribution heat losses 10 - 20 % • Customer losses 5 - 10 % Figure 2: heat balance in district heating system. Source: Zinko, H., 2008 A successful district heating system requires both a cheap local energy source and a heating market. For example, the most used energy sources in Nordic countries are: waste heat from power generation in Denmark and Finland, geothermal energy in Iceland, waste heat from incineration in Norway and a mixture of several energy sources in Sweden. [Nordvärme, 1996] District heating systems have been built all over the world. In Europe, DH has been introduced into many countries.
Some of them are mentioned as follows. In Sweden, DH has reached a significant position with a high penetration in the heat market. The total DH production is
2. Literature study 4 estimated to 50 TWh per year. It is produced by CHP plants using a mixture of heat sources in order to reduce fuel oil dependence; main fuels are coal, wood chips, peat, heat from waste incineration and waste heat recovered from industrial processes. In Austria, the largest district heating system is in Vienna producing 5163 GWh per year in 2005, using 22 % heat from waste incineration and 72 % from waste heat from municipal power plants and large industrial plants. In Denmark, DH has been installed in the main cities; it produces more than 60% of space heating and water heating by generating 82.4% of it in CHP plants.
In Finland, district heating represents 50% of the total heating market, produced mainly by cogeneration plants. In Germany, the largest DH network is located in Berlin. The total connected heat load is around 52.729 MW and 83% comes from CHP plants using mainly natural gas and coal. In Iceland, DH has the highest penetration covering 95% of the demand and it is produced from geothermal energy. Other countries that use district heating are Italy, Norway, Russia, Serbia and the United Kingdom. Moreover, district heating is also used in the United States, Canada and Japan. [Wiki DH, 2009] 2.1.2.
District cooling Nowadays, cooling demand requires much more energy than heating. Hence, district cooling has been developed quickly. DC networks operate in the same way as district heating networks. Water is chilled centrally and then distributes to multiple buildings through an underground pipe network. This cold water is used in cooling processes of industry or to create thermal comfort in residential housing, shops or offices. The cooling is provided from a cooling plant where it is centrally produced in an efficient way. In this way, it is possible to eliminate the need for individual traditional air conditioning systems in buildings.
[Danfoss, 2009] Chilled water supplied by DC systems may be produced by different techniques. The most common ways are compression cooling and absorption cooling. Usually, a mix of techniques is used to get the optimal solution. In Europe, several sources are used to produce cold water, such as cold sea or lake water, but also DH and industrial waste heat that fuel absorption machines [Danfoss, 2009]. A competitive way is to use heat produced by CHP plants with high combined efficiency. So, thermal energy turns into cold water by means of absorption cooling chillers. It is also useful to take advantage of heat production during hot seasons.
[Nccc, 2009] Basically, a DC system consists of three main components: the central plant including power generation, cooling equipment and thermal storage; the distribution network and the consumer system which is composed of air handling units or fan coils and chilled water piping in the buildings. [Nccc, 2009] Furthermore, district cooling offers a good opportunity to reduce electricity consumption; decreasing it by more than 65% compared to traditional air conditioning systems. As well as being a technology with minimum environmental impact that uses energy efficiently and with low air emissions.
[Danfoss, 2009] 2.1.3. Environmental effects District heating and cooling system are thought to have a big potential to reduce greenhouse gas emissions. This importance has been accepted in many member countries where they are introducing these types of systems as well as other technologies to reduce environmental impact. It provides an attractive solution for meeting environmental targets established by the Kyoto protocol.
2. Literature study 5 District systems offer an excellent opportunity for reducing environmental pollution and saving energy. These systems let one use flexible technology such as combined heat and power which works with any fuel including renewable energy resources or industrial waste energy. [IEA DHC, 2009] 2.1.4. Situation of district energy system in Spain Because of Spain characteristics, the first district heating networks are starting to be developed and they are mainly fuelled by biomass. They can be utilized to supply heat during the cold period and to refrigerate in summer. At present, there are only a few small district heating systems in Spain because DH systems are not commonly used as a heat production solution.
Some of them are going to be explained in this section. In the last part, the development of a new project based on heating and cooling production is mentioned. • Cuéllar Project (Segovia) Cuéllar is a 10.000 inhabitant town sited 120 km north of Madrid, with numerous wood industries in the area. The district heating system is based on a 5.95 MWth plant with two wood waste boilers. It supplies hot water for heating and sanitary water by using the waste from those mills. The system supplies district heating to more than 1000 inhabitants (15 detached houses, 5 blocks of flats and some municipal buildings: a sports’ centre, a library and a public school for 500 pupils).
The DH plant is fuelled by waste recovered from forest cleaning as well as other types of biomass because the boilers are prepared to use bark, saw wood or plywood as fuel indistinctly. In this case, the use of wood waste coming from nearby industries (furniture, boards and other wood mills) facilitates a cheap energy source to feed both boilers: a 5,25 MW unit for winter heating and a 0,7 MW unit for sanitary water in summer. The DH is distributed to the users through pre-isolated piping, with a heat counter in each one for municipal invoicing.
The plant is maintained by the city council with the collaboration of Valladolid University, whose technical department gets and analyses the results, advising IDAE and EREN about eventual corrections and improvements in the system. The main aim of the IDAE was to study the feasibility and real exploitation conditions for this type of plant, to produce heating in cold areas of Spain, where DH systems are not commonly used to produce heating. The project implementation took 9 months, and started to operate in March 1999. [Heymo, 2009] Figure 3: DH system in Cuéllar. Source: Heymo, 2009
Literature study 6 • Sant Pere de Torelló Project (Barcelona) The first district heating system was built in 1993 in Sant Pere de Torrelló (Barcelona). It is a biomass plant that consumes between 6.000 and 7.000 tons per year of biomass. The system supplies district heating to 540 of the 800 homes in the municipality. Figure 4: DH system in Sant Pere de Torelló. Source: Solé, 2009 In the future, this system might be substituted by a new cogeneration plant because the old one is inefficient both from the point of view of energy and economy. The CHP plant will produce district heating and electricity to meet the demand of the area, which is estimated to be 4.5 MWe.
The total power generated will be 5.4 MWe. [Biomasa, 2009] [Solé, 2009] • Molins de Rei Project (Barcelona) This district heating system was built in Molins de Rei (Barcelona). It is based on a mobile grate boiler of 2 MW and a biomass boiler of 1.7 MW which is fuelled by 3.000 tons of wood waste per year. It reduces the production of 3.000 tons of CO2 annually. The system produces heat to supply hot water and heating to 700 dwellings. [Biomasa, 2009] [Solé, 2009] Figure 5: DH system in Molins de Rei. Source: Solé, 2009
2. Literature study 7 • Tub verd Project - Mataró (Barcelona) This system started working in 2003 and it supplies heat to several schools, the Mataró hospital, the municipal swimming pool and a municipal sports’ centre which uses absorption chillers during summer. In the near future, the system will meet the demand of several dwellings. The total thermal power produced was 4.9 MWth in 2006. The system consists of a Natural Gas engines CHP with an installed power of 6 MWe and furthermore, it recovers the waste heat from waste mud drying. It is believed that the system will supply 12.000 MWh of heat per year and it will prevent the production of 500 tons of CO2.
The cost of the system is estimated to be 3.630M€. [Biomasa, 2009] [Solé, 2009] Figure 6: DH system in Mataró. Source: Solé, 2009 • Geolit Project (Jaén) This project will be the first project developed in Spain that will produce hot and cold water. The project is being developed in a Science and Technology Park in Mengibar (Jaén) and it will cost 2,3 M€. It is a project that promotes the use of renewable energy sources and applies energy efficient measures. The system is going to supply district heating and district cooling to the buildings of the Park with an estimated total surface of 33.000 m2 .
The energy will be supplied by two biomass boilers with a power of 3 MW each. The boilers will be fuelled by more than 800 tons of biomass per year. The fuel used is olive chips, wood waste and other types of biomass. The cooling will be produced by an absorption cooling plant [Biomasa, 2009]. 2.2. Cogeneration and trigeneration 2.2.1. Cogeneration Cogeneration (also known as Combined Heat and Power or CHP) is the process in which electricity and heat is produced simultaneously, both used thereafter in different applications. The fundamental principle of cogeneration is to maximise efficiency of systems by obtaining as much final energy as possible from a fuelled plant.
CHP can substantially increase the efficiency of energy utilization, resulting in lower operating costs for the user. Systems should be designed according to the heat demand of the application. This can be an individual building, an industrial plant or a district heating and cooling system for a city. [Cogen, 2009]
2. Literature study 8 By producing heat and electricity at the same time, the efficiency of a cogeneration plant can reach up to 90%. Therefore, CHP offers a big potential of energy saving which represents between 15 and 40% compared to conventional systems. Traditionally, electricity and heat was generated from separate conventional power stations and boilers. CHP uses the heat that would be wasted in a conventional power plant to produce electricity. Consequently, less fuel is consumed to produce the same amount of useful energy [Unep, 2009]. Figure 7 and Figure 8 illustrate the differences of power generation in a cogeneration system compared to a conventional system.
Figure 7: Typical cogeneration scheme Source: Unep, 2009 Figure 8: Typical conventional power generation scheme. Source: Unep, 2009 The benefits that cogeneration offer society by optimising the energy supply are important. Cogeneration is the most efficient way of power generation reducing environmental emissions, in particular CO2 which is the main greenhouse gas. So, it becomes a helpful solution to achieve the Kyoto Protocol targets. Moreover, cost savings are significant, providing additional competitiveness for industries and domestic users. It offers also an opportunity to decentralise electricity generation, providing that plant is designed to meet the needs of local consumers with high efficiency, reducing transmission losses and increasing flexibility and security of supply.
In addition, cogeneration reduces the import dependency of fuels and promotes liberalisation in energy markets. [Educogen, 2001] End users with significant thermal and power needs can generate both thermal and electrical energy in a single CHP system. Heat is generally recovered in the way of steam or hot water. However, in some cases it can be used directly for applications such as heating or drying processes. Apart from this, the waste heat can be utilized to drive equipment that is thermally activated, such as absorption chillers for cooling. [Cogen, 2009b] Important characteristics for CHP technology are: • Annual operating hours: typically more than 6 000 • High thermal and power output resulting in high overall efficiency • Low maintenance costs • Low emissions • High reliability
2. Literature study 9 Traditionally, CHP systems have been applied to larger industries with high steam and power demands such as chemicals, paper or refining but also for institutional applications such as universities and hospitals. Currently, a large potential can be found for smaller CHP systems in light industrial or commercial and residential applications. Some important requirements to consider before deciding to install a cogeneration system are: great amount of heat consumption, fuel supply reliability and high utilization factor (>5 000 hours/year). [Unep, 2009] Figure 9: CHP plant in Winnington.
Source: http://www.flickr.com A cogeneration system can be classified according to three points [Lozano, M.A., 2008]: 1. Process sequence in heat and electricity production: - Bottoming cycle: heat is produced first - Topping cycle: electricity is produced first 2. Power generation capacity: - Micro CHP: 50 MWe 3. Depending on the type of motor machine. - Steam turbine - Gas turbine - Reciprocating engine - Combined cycle - Micro turbines - Fuel cells - Stirling engines
2. Literature study 10 2.2.2. Trigeneration Trigeneration is a particular case of cogeneration in which waste heat from processes is utilised to generate cooling by means of absorption chillers. It is the simultaneous production of three types of energy: cooling, heating and electricity, by using only one fuel input. Trigeneration is also known as CHCP that stands for combined heating, cooling and power generation. A typical trigeneration plant can be described as a cogeneration plant that has added absorption chillers. The system converts waste heat from the CHP plant into chilled water. Four forms of energy can be obtained from this process: hot water, steam, chilled water and electricity.
[Trigen, 2009b] Figure 10: Typical trigeneration system. Source: GEenergy, 2008 Trigeneration systems offer some advantages over conventional systems. The main factors that support its development are global efficiency increase, reliability of energy supply and decrease of annual costs. Generally, the global system efficiency of a trigeneration power plant reaches values from 86% to 93% compared to 33% of a typical central power plant. Besides, it is also more efficient than CHP plant. [Trigen, 2009] Figure 11: Energy flows in a trigeneration system. Source: Trigen, 2009
2. Literature study 11 Cooling technology enables CHP plants to be used during warmer season, increasing equipment utilization and reducing amortization period. Moreover, trigeneration is promoted by The European Union because it is thought that it can help to reduce climatic change. It is the most environmental friendly method of generating power and energy, especially when the fuel source is Biodiesel or Biomethane. [Trigen, 2009] Trigeneration systems are usually installed in hospitals, universities or groups of residential and offices buildings. In this case, it is also referred to as a "district energy system" or "integrated energy system".
This possibility for supplying both heating and cooling for buildings is really interesting, because these systems offer greater operational flexibility. This is particularly relevant in countries where buildings need to be air-conditioned and industries require process cooling. [Trigen, 2009] There are different possibilities for refrigeration [GEenergy, 2008]: • Absorption chillers: - Operation with hot water - Operation with steam - Direct heat through combustion • Compression-type refrigeration machines: - Direct drive power - Electrical drive power 2.2.3. Efficiency parameters of cogeneration Systems It is necessary to define several parameters in order to value investment opportunity, select the most adequate system and optimize its operation.
Then, the cogeneration system is supposed to be a black box in order to simplify the system. It consumes F units of fuel (lower heating value - LHV) and produces W units of power and Q units of heat simultaneously. This system is compared to a conventional system producing the same amount of heat and power separately. [Lozano, M.A., 2008] Figure 12 shows the energy flows in a cogeneration system and in a conventional system to be compared.
Figure 12: Comparison between a cogeneration system and a conventional system. Source: Lozano, M.A., 2008 The fundamental parameters that determine the energy performance of the system are:
2. Literature study 12 Electric efficiency RWF = W/F Thermal efficiency RQF = Q/F Global efficiency ηe = (W + Q)/F Power Heat Ratio RWQ = W/Q Being F the primary energy fuelled, W the electricity generated and Q the heat produced that meet the energy demand (no waste heat included). Moreover, other parameters which are useful to compare different cogeneration systems to conventional systems are defined below.
Primary energy saving ∆F = F* - F =F W+ FQ –F = W/ηW + Q/ηQ – F Fuel energy saving ratio FESR = I∆F = ∆F/F* = 1 – F/ [W/ηW + Q/ηQ] Equivalent electric performance REE = ηee = W/(F - FQ) = W/(F – Q/ηQ) FESR and REE are used to find out if cogeneration systems are more efficient than conventional system producing heat and power.
The last parameter REE must higher than the minimum equivalent electric performance REEmin to be able to receive an extra income when electricity is sold. It is established by Special Regime in “Royal Decree 661/2007”. [Lozano, M.A., 2008] 2.2.4. Environmental and economic advantages Cogeneration systems are highly efficient systems and offer a real opportunity to save primary energy and reduce fossil fuel usage; resulting in a reduction of greenhouse gases emissions and energy costs. CHP plants also improve security of energy supply and decrease risks associated with rapidly rising electricity prices.
Besides, transmission and distribution losses can be reduced when many smaller CHP plants are constructed near consumption areas. [EnerG, 2009] Cogeneration reduces environmental impacts and helps to achieve Kyoto Protocol targets, decreasing CO2, NOx and SO2 emissions. In Spain, it shows a saving of primary energy of 850.000 toes per year, which means 3% of the total natural gas imports. It also prevents the production of from 7Mton of CO2 per year. In conclusion, without using cogeneration Spain will break the Kyoto Protocol targets by 5% more than at present. [Acogen, 2009] 2.2.5. Development of cogeneration in Spain In Spain, cogeneration systems had the greatest and widest use at the end of the XIX century and the beginning of the XX century.
Some years later, the development of the electrical sector meant that big power facilities and distribution networks were built. Therefore, cogeneration systems were moved to medium-sized industries.
Nevertheless, this situation started to change between 1973 and 1979. Oil dependency problems forced the authorities to promote efficient energy use. Some research projects were done and the Government gave help to support investment, financing and electricity sales. This development was also due to the availability of natural gas in Spain. It was a clean and easily used fuel; hence, supply systems were reconverted to use the new fuel. On the other hand, deregulation of energy markets provided electricity purchase and sale. The two last factors that promote a CHP system were increasing environmental concerns and cogeneration technology development.
[Lozano, M.A., 2008]
2. Literature study 13 In Spain, cogeneration is located mainly in industrial areas. The distribution of cogeneration capacity in Spain in 2006 is shown in Table 1. Table 1: Cogeneration capacity in Spain’s industrial areas. Source: Viladrich-Grau, M., Vila, J., 2007. AREA CAPACITY (MW) NUMBER OF PLANTS Catalonia 1194 138 Andalusia 701 49 Galicia 595 84 Valencia 579 141 Castilla-Leon 509 55 Cogeneration plants are principally built in industries with intensive energy use. The presence of cogeneration in the tertiary sector only represents 7.4% due to the low residential heating hours required during the year compared to other European countries; although it is thought that the cogeneration combined with refrigeration (trigeneration) is going to be well- developed in this sector because a number of social changes in Spain have increased the cooling demand in the tertiary-residential sector.
Accordingly and as a response to these needs cogeneration will expand. [Viladrich-Grau, M., Vila, J., 2007] The distribution of cogeneration systems in Spain divided by sectors is gathered in Table 2. Table 2: Distribution of cogeneration by industry sectors. Source: Viladrich-Grau, M., Vila, J., 2007. SECTOR INSTALLED POWER (MW) NUMBER OF SYSTEMS PERCENTAGE (MW by industry sector) Agro-alimentary industry 1057 138 18.2 Chemical industry 944 54 16.3 Paper mill industry 876 75 15.1 Oil refinery 577 11 9.9 Non metal ore industry 536 157 9.2 Other industries 581 83 10.0 Textile industry 412 61 7.1 Buildings and services 432 98 7.4 Other 388 63 6.7 TOTAL 5803 740 100 According to the primary energy used, in 2006, natural gas represented 72% of cogeneration plants and 64% of the available capacity.
Secondly, liquid fuels corresponded to 25% of available capacity and the main fuels are oil derivatives such as diesel, gasoline, fuel oil or refinery gas. The rest of fuels had limited significance; they fuelled 3% of plants and represented 11% of available capacity. [Viladrich-Grau, M., Vila, J., 2007] In 2007, the most commonly used fuel in power CHP system was natural gas which represents over 80%, followed by fuel oil or gas oil with 10%. The distribution of fuels used in CHP system is shown in Figure 13.
2. Literature study 14 Fuels used in CHP systems 81% 10% 4% 4% 1% Natural gas Fuel oil / Gas oil Other fuels Refinery gas High oven gas Figure 13: Distribution of the fuels used in CHP systems. Source: Cogen, 2009 When cogeneration plants are studied from the point of view of installed capacity, it is seen that more than 50% of plants have a capacity of between 1 and 5 MW. Hence, Spanish cogeneration profile is made up of many small and low-capacity CHP plants. This fact may explain why cogeneration is not currently considered a real option for the production of electricity on a large scale.
The main objective of these small plants is often to reduce energy costs for the companies, instead of producing electricity.
The technologies most frequently employed for cogeneration are, combined cycle, condensed steam turbines, internal combustion engines and gas turbines with counter-pressure heat recovery. [Acogen, 2009] The cogeneration installed power has been increasing rapidly and it was 6075 MW from 874 CHP plants in 2007. [CNE, 2009] The evolution of the installed power and electricity sold in the market can be seen in the Figure 14. Figure 14: Evolution of the cogeneration installed power in Spain. Source: Cogen, 2009
2. Literature study 15 In Figure 15, the increase in the number of CHP plants installed in Spain can be seen.
Figure 15: Evolution of the number of CHP plants in Spain. Source: Cogen, 2009 At present, the production of electricity from cogeneration systems represents over 11% of the total electricity generated. This evolution is shown in Figure 16. Figure 16: Percentage of the electricity production from CHP systems. Source: Cogen, 2009 At present, the characteristics of the cogeneration in Spain are listed as follows: • More than 6.000 MW of installed power • Approximately 900 CHP plants distributed around Spain • It creates more than 4.500 direct employment positions and more than 10.000 indirect employment positions • It has a turnover of more than 3.800 M€ per year
2. Literature study 16 • The total electricity fed into the public grid is 19.700 GWh per year • The total electricity generated is estimated to be over 29.700 GWh per year • It covers almost 10% of the electricity demand of Spain • Primary energy saving of 850.000 toe per year, which means 3% of the total natural gas imports. • Water saving of 40 million of m3 per year. • It prevents the production of some 7 million of tons of CO2 per year. In general, the electricity is produced by different types of fuels. Table 3 gathers the total electricity generated from the diverse fuels and the annual variation.
[Energía, 2008] Table 3: Total electricity production in Spain. Source: Energía, 2008 TOTAL ELECTRICITY PRODUCTION (GWh) 2006 2007 VARIATION (%) HYDRO 25 330 26 447 4.4 FOSSIL FUELS 150 737 158 239 5.0 NUCLEAR 60 126 54 982 -8.6 Ordinary Regime 236 193 239 668 1.5 % RENEWABLE AND WASTE 33 069 37 845 14.4 COGENERATION 34 418 35 043 1.8 Special Regime 67 487 72 888 8 TOTAL 291 045 300 146 3.1 The distribution of fuels used to produce electricity is represented in Figure 17. 87% of the electricity is generated in Ordinary Regime by hydro, fossil fuels and nuclear; production in Special Regime only represents 12% and this electricity comes from renewable sources, waste or cogeneration.
In Spain, electricity is mostly produced from fossil fuels which correspond to 60% of the total. [Energía, 2008] Electricity production 37% 4% 8% 1% 11% 16% 18% 5% Combined cycle Wind Other:CHP,biomass, minihydro,w aste… International exchange Hydro Nuclear Coal Fuel/Gas Figure 17: Distribution of the fuels used to produce electricity in December 2007. Source: Data from Energía, 2008
2. Literature study 17 The evolution of the electricity which is sold in Special Regime can be observed in Table 4. Generally, it has been increasing annually, which means that people have started to become concerned about environmental issues. Table 4: Evolution of the electricity sold in Special Regime. Source: Energía, 2008
2. Literature study 18 2.3. Combined Heat and Power technology This section describes the current status of some natural gas-fired distributed energy resource technologies. The power technologies analysed can be used in CHP plants.
Despite being capable of utilizing a variety of fuels in a range of applications, these technologies are evaluated according to electric power and combined heat and power (CHP) applications using natural gas [Nrel, 2003]. The technologies characterized can meet the needs of a wide range in the residential, commercial and industrial sectors. Comparative performance and costs of each technology option must be taken into consideration in order to decide the best alternative. Table 5 compares the characteristics of each technology.
Table 5: Comparison of CHP technologies. Source: Nrel, 2003 • Reciprocating Engines Reciprocating internal combustion engines are a widespread and developed technology for power generation. They can be used for all types of power generation. Spark ignition engines for power generation use natural gas as the preferred fuel but they can run on propane or gasoline. However, compression ignition engines operate on diesel fuel or heavy oil, moreover they can run in a dual-fuel configuration burning primarily natural gas with a small part of diesel fuel. The advantages of using this technology are several: low investment cost, easy start-up, reliability when properly maintained and good load-following characteristics.
2. Literature study 19 other hand, some drawbacks can have relatively high noise levels, rather high air emissions, and regular maintenance required. Gas-fired reciprocating engines are well suited for CHP in commercial and light industrial applications of less than 5 MW. Smaller engine systems may be used to produce hot water and larger engine systems are designed to produce steam at low pressure. [Nrel, 2003] • Gas Turbines Gas turbines are an established technology in sizes from several hundred kilowatts up to about 50 MW. They produce high-quality heat which is used to generate steam. This steam usually runs a generator to produce electricity, it is known as combined-cycle configuration.
Gas turbines mainly burn natural gas, but also they can be fuelled by a variety of oil fuels or have a dual-fuel configuration. An important advantage of gas turbines CHP systems is the high- quality of waste heat available in the exhaust gas, apart from which maintenance costs are one of the lowest. That is why gas turbines are a great option for industrial or commercial CHP applications larger than 5 MW. This CHP technology is also useful in many industrial processes where the high-temperature exhaust gas is appropriate for generating steam at high pressure. By running a gas turbine in simple cycle, the hot exhaust gas may be employed directly in a process of the industry or used to generate steam and hot water by adding a heat recovery steam generator.
[Nrel, 2003] • Steam Turbines Steam turbines are one of the most versatile and oldest main technologies used to drive generators and mechanical machinery. What’s more, steam which runs the turbine is extracted to be utilized directly in a process or to be converted to other types of thermal energy such as hot water or chilled water that may be used for district heating and cooling. The capacity of steam turbines can reach several hundred MW for large utility power plants. Steam turbines are run by high pressure steam which must be produced in a boiler or heat recovery steam generator. Boilers can be fuelled by different varieties of fuels, including natural gas, fossil fuels such as coal and oil or renewable fuels like wood or municipal waste.
Steam turbines CHP systems are mainly used in industrial processes where solid or waste fuels are available to be burnt in boilers. [Nrel, 2003] Figure 18 illustrates the range in which the mentioned technologies operate as well as power heat ratio achieved.
Figure 18: Selection of the cogeneration system. Source: Modified from Lozano, M.A., 2008
2. Literature study 20 In Spain, the most installed technologies in cogeneration systems are gas turbines and internal combustion engines. Gas turbines produce electricity during 80% of the year, while engines have an average functional time of 48% and they represent 42% of the total capacity. Once the system is in operation, the cogeneration production costs for gas turbines are lower than for engines. Nevertheless, internal combustion engines have less start-stop difficulties than gas turbines.
Hence, they are more flexible, being turned on and off to meet the demand requirements much faster and with less cost than turbines [Viladrich-Grau, M., Vila, J., 2007]. 2.4. Cooling techniques: Compression and absorption systems 2.4.1. Compression cooling Electrical compressor chillers are the most widespread type of technology used to create cooling. It is used to produce air conditioning and refrigeration in buildings, industries and automobiles.
The main components of the simple compression system, illustrated in Figure 19, are a compressor, a condenser, an expansion valve and an evaporator. Figure 19 : Simple compression cycle . Source: Modified from Ruiz, 2007. The compressor is used to increase the pressure and the temperature of the refrigerant vapour from the evaporator to the condenser. The cooling capacity of the machine is regulated by changing the output temperature of the compressor. The refrigerant vapour at high pressure is cooled in the condenser by means of air or water, and it becomes a liquid. Then, it goes through an expansion valve to reduce the pressure in the evaporator.
In the evaporator the cooling effect takes place. The refrigerant liquid evaporates by taking heat away from the space. The evaporator receives two different names according to its operation; when the air of the space that needs to be chilled is cooled directly by the evaporator it is called air coil. But if the evaporator cools a liquid that is heat exchanged with the space air it is called a chiller. [Fernández, P., 2008a] To produce the cooling effect it is necessary for the temperature of the refrigerant in the evaporator to be lower than the cooled space and in the condenser to be higher than the environmental temperature.
In larger systems, water which extracts heat from the condenser is usually cooled in a cooling tower. However, in smaller systems it is possible to use air to remove heat from the refrigerant.
2. Literature study 21 The energy analysis of the cycle is explained by using enthalpy (kJ/kg). The heat removed from the space in the evaporator known as cooling capacity is QL =h6-h5 The work made by the compressor is WC = h2-h1 The heat extraction in the condenser is QH = h3-h4 Thus, it possible to define the coefficient of performance as 1 2 5 6 h - h h - h COP = However, the real cycle has some losses due to mechanical inefficiencies of the compressor or energy losses in heat transmission and pressure drops that decrease the efficiency of the system.
Besides, it is important to select the type of compressor according to the type of refrigerant chosen.
There are different types of compressor. For example, centrifugal compressor work better when pressures are low and specific volumes are high; on the other hand, reciprocating compressors work better at higher pressures and smaller specific volumes. Other types of compressor are screw, scroll and vane compressors. [Wulfinghoff, 1999] A simple description of the energy flows in a compression machine (Centrifugal type water chillers) is shown in Figure 20.
Figure 20 : Energy flows in a typical compression machine. 2.4.2. Absorption cooling The main characteristic of an absorption cooling system is that it uses heat energy as fuel in order to produce cooling. For this reason, it is common to use these systems in plants that have an excess power capacity during the summer season, so it is possible to take advantage of the installed power of the systems used to produce heat. The components of an absorption machine are integrated closely within a single compact package. The most used pairs of refrigerant-absorber are the system ammonia-water and system water-salt lithium bromide.
There are some differences between chiller models according to the heat source and the number of stages. Initially, absorption chillers were fuelled by steam or hot water at high temperature but currently direct-fired absorption chillers are the most used because they have an integrated boiler reaching higher efficiency levels. Furthermore, single-stage chillers have been replaced by two-stage machines or even multiple-effect ones that provide significantly higher efficiency. 0.17 1 1.17 Electricity Cooling water Cold water